Coxsackievirus B3 causes myocarditis, pancreatitis and meningo-encephalitis but, despite the resulting human morbidity and mortality, neither a treatment, nor a vaccine, is available. My lab has shown that the metabolic status of the host cell plays a key role in determining the outcome of CVB3 infection and, in the previous period of support, we identified stem cells as early targets of CVB3 infection. In this renewal application, I propose 4 Specific Aims, focusing on the following topics : 1. Bone marrow is the main repository of stem cells in the adult, and we show herein that CVB naturally infects ~1% of bone marrow cells in vivo. We shall identify and characterize the bone marrow cells that become infected; and will evaluate the biological implications of this infection. 2. We shall determine the role of cellular activation in regulating CVB3 infection in the heart. We shall use a variety of methods to ask: are proliferating cells targeted? Are myocardial stem cells a preferred site of infection? Does prior myocardial damage alter viral replication in the heart, and does this exacerbate the viral myocarditis? 3. The innate immune response to picornaviruses in general, and to enteroviruses in particular, is poorly understood. We shall investigate the innate responses to CVB3 infection in lymphoid tissues (spleen & lymph nodes). What responses are mounted? Which of the many innate molecular sensors are involved? How does activation of the innate system affect the outcome of subsequent CVB3 infection? 4. Many virus infections induce very strong T cell responses, but CVB3 appears not to do so; neither CD4+ nor CD8+ T cells are strongly activated during wtCVB3 infection. We shall use novel methods to map, kinetically and anatomically, the presentation of CVB3-encoded MHC class I & class II epitopes, and will ask how the innate responses to CVB3 infection affect the subsequent development of adaptive T cell immunity.